A form-and-seal unit for producing aseptic sealed packages from a tube of packaging material filled with a pourable food product. The unit comprises a fixed structure and two forming assemblies which interact alternately and cyclically with the tube of packaging material, and have respective pairs of jaws movable between an open position and a closed position in which they grip the packaging material tube between respective sealing means, and respective forming flaps selectable from different types of forming flaps for producing different types of packages, and having respective half shell forming portions which surround the tube of packaging material to determine the volume and shape of the packages. The approach movement of the forming flaps towards the packaging material tube is controlled by a fixed cam mechanism having different work profiles selectively engaged by cam-follower rollers of the forming flaps depending on the type of forming flaps used.